Your entire premise is based on if it isn’t codified in a book therefore it must not exist. 

But in your survey you dismiss the copious amounts of books and translations on the very subject. 

For instance:
The wuzhen pian  https://www.amazon.co.uk/Foundations-Internal-Alchemy-Taoist-Practice/dp/0984308253/?_encoding=UTF8&pd_rd_w=pCDVW&content-id=amzn1.sym.b23cb387-9a65-4c5f-8523-6979ddb19d2e&pf_rd_p=b23cb387-9a65-4c5f-8523-6979ddb19d2e&pf_rd_r=7GR5KN7H40MWQYXWWXVM&pd_rd_wg=dq9NQ&pd_rd_r=2f21dd76-cfac-4bb9-977a-1f4166c9977e&ref_=aufs_ap_sc_mbl

  The xiuzhen boubian  https://www.amazon.co.uk/Cultivating-Tao-Internal-Alchemy-Masters/dp/0985547510/?_encoding=UTF8&pd_rd_w=pCDVW&content-id=amzn1.sym.b23cb387-9a65-4c5f-8523-6979ddb19d2e&pf_rd_p=b23cb387-9a65-4c5f-8523-6979ddb19d2e&pf_rd_r=7GR5KN7H40MWQYXWWXVM&pd_rd_wg=dq9NQ&pd_rd_r=2f21dd76-cfac-4bb9-977a-1f4166c9977e&ref_=aufs_ap_sc_mbl

The world upside down
https://www.amazon.co.uk/World-Upside-Down-Internal-Alchemy/dp/0984308261/?_encoding=UTF8&pd_rd_w=pCDVW&content-id=amzn1.sym.b23cb387-9a65-4c5f-8523-6979ddb19d2e&pf_rd_p=b23cb387-9a65-4c5f-8523-6979ddb19d2e&pf_rd_r=7GR5KN7H40MWQYXWWXVM&pd_rd_wg=dq9NQ&pd_rd_r=2f21dd76-cfac-4bb9-977a-1f4166c9977e&ref_=aufs_ap_sc_mbl

And myriads of others if you really look at the older Chinese anthropological texts often found in caves and such. Secondly as pointed out the method is often taught orally. I have yet to see a text to describe actual methods across the different stages a person will experience in either Chinese or in English. Sufficed to say you will never find Taoist Texts to outline nei dan / nei gong processes and results like the texts for qi gong (which again are plentiful)  
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